Systems and Methods for Error Correction in DNA Sequencing

ABSTRACT

Disclosed are systems and methods for polynucleotide sequencing where detection and correction of base calling errors can be achieved without reliance on a reference sequence. In certain embodiments, redundant information can be introduced during measurement so as to allow such detection of errors. Such redundant information and measurements can be facilitated by encoding of nucleotide sequence being measured. Various examples of such encoding, redundancy introduction, and decoding are provided.

RELATED APPLICATION(S)

This application claims priority pursuant to 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 61/254,545, entitled “ErrorCorrecting Codes Used in Sequencing Ligation,” filed on Oct. 23, 2009,the entirety of which is incorporated herein by reference.

FIELD

The present disclosure generally relates to the field of DNA sequencingincluding systems and methods for detection and correction of errors orambiguities encountered in or associated with sequencing of DNA samples.

BACKGROUND

In certain DNA sequencing systems, identities of nucleotides of a samplecan be determined by identifying complementary nucleotides thathybridize to or pair or otherwise match with nucleotides of the sample.One or more of such complementary nucleotides may be part of a probe orprobe set that can be used to test or interrogate the sample nucleotidesequence.

Typically, probes include a detectable feature such as chemical orphysical features that can be identified under suitable conditions. Asan example, dyes that fluoresce or otherwise emit an optical signalunder suitable conditions can be used as detectable features. Bydetecting the feature (e.g., the fluorescence of a dye), informationabout the probe, and thus the portion of the sample where the probehybridizes, pairs, or matches can be obtained.

Errors and ambiguities can be introduced or otherwise occur at or duringvarious stages of sequencing and sequencing-related operations andprocesses. In certain situations, it can be impossible to even know thatan error has occurred or an ambiguity exists. While it may in somesituations be possible to resolve ambiguity or distinguish an error fromcorrect but unusual or unexpected sequence information such as singlenucleotide polymorphism, determining whether the sequence information isambiguous, correct, or erroneous can typically only be detected bycomparison of the sequence information with a reference. Further, evenif the putative sequencing error or ambiguity is identified as a trueerror or ambiguity, there is often no mechanism or capability to correctthe error or ambiguity without having to repeat some or all of themeasurements.

SUMMARY

The present disclosure relates generally to methods for determiningsequence information for nucleic acid samples but can also haveapplicability to determination of sequence information for otherbiopolymers such as, for example peptides or proteins. The presentdisclosure also relates generally to the detection, identification,resolution, and/or correction of errors and ambiguities in sequenceinformation.

Using nucleic acids as an example, without limitation, some embodimentsconfigure a template polynucleotide so as to allow testing, observing,or interrogating of one or more nucleotides, the number of nucleotidesrepresented by the shorthand “K.” The value of “K” is not limited to anyparticular range. Methods can further include testing, observing, orinterrogating one or more of the K nucleotides so as to yieldmeasurements of one or more detectable characteristics, the number ofdetectable characteristics represented by the shorthand “M.” The valueof “M” is not limited to any particular range. Measurements can alsoinclude data representative of one or more of the K nucleotides and alsoinclude redundant data that can be used for error or ambiguitydetection.

In some embodiments, redundancy can be achieved by, for example, havinga quantity N^(M) greater than a quantity L^(K), with each of the Knucleotides being one of L types, and with each of the M detectablecharacteristics being one of N types. The values of “L” and “N” are notlimited to any particular range. In some embodiments, the quantity Lincludes quantity of 4 corresponding to nucleotide types A, C, G, and T.In some embodiments, redundancy can be achieved by selecting thequantity N and/or by selecting the quantity M.

In some exemplary and non-limiting embodiments, the quantity M can berepresented as M=K*S/P where S represents a number of uniquehybridization, pairing, matching, interrogation, or probing steps and Prepresents a number of variable factors associated with one of more ofthose steps. Redundancy can be achieved by selecting the quantity Sand/or by selecting the quantity P.

The present disclosure also provides methods involving decoding orinterpretation of measurements to assist in determining whether ameasurement or set of measurements includes any errors or ambiguities.In some embodiments, the method can include performing an errorcorrection or ambiguity resolution based on one or more detected errorsor ambiguities and one or more redundant data points. The presentdisclosure provides error detection and/or correction or ambiguitydetection and/or resolution that does not require a reference sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a preferred embodiment of the presentlydisclosed system;

FIG. 1B is a block diagram of some embodiments of a DNA sequencingsystem having a component that allows detection and or correction oferrors and/or ambiguities;

FIG. 2A is another block diagram of some embodiments wherein the erroror ambiguity detection and correction component of FIG. 1B can include afeature where data associated with DNA sequencing can be encoded so asto introduce redundancy suitable for detection and correction of errorsand/or ambiguities;

FIG. 2B is another block diagram of some embodiments wherein the errorand/or ambiguity detection and correction component of FIG. 1B caninclude a feature where the coded measurements having the redundancy canbe decoded so as to allow extraction of the data, as well as informationfor detection and correction of errors and/or ambiguities;

FIG. 2C is another representation of some embodiments of the presentlydisclosed accuracy-enhancing encoding/decoding scheme;

FIG. 3 is a representation of some embodiments where redundancy can beintroduced to a measurement by interrogating a selected sequence ofnucleotides a number of times at different offsets between thenucleotide sequence and interrogating probes;

FIGS. 4A and 4B are representations of some embodiments where redundancycan be introduced to a measurement by configuring the probes withdetectable dyes in various manners;

FIG. 5 is a representation of shows some embodiments where redundancycan be introduced to a measurement by interrogating the same portion ofthe nucleotide sequence two or more times using differently configuredprobes;

FIG. 6 is a flow-chart of some embodiments wherein a process can beimplemented to configure interrogating probes and/or manner ofinterrogation so as to introduce redundancy;

FIG. 7 is a representation of some embodiments where redundancy can beintroduced to a measurement by configuring the probes with detectabledyes in various manners;

FIG. 8 is a flow-chart which shows a process that can be implemented tofacilitate the example configuration of FIG. 7;

FIG. 9 is another representation of some embodiments where redundancycan be introduced to a measurement by configuring the probes withdetectable dyes in various manners;

FIG. 10 is a flow-chart which shows a process that can be implemented tofacilitate the example configuration of FIG. 9;

FIG. 11 is a representation of some embodiments where redundancy can beintroduced to a measurement by interrogating the same portion of thenucleotide sequence two or more times using differently configuredprobes;

FIG. 12 is a flow-chart which shows a process that can be implemented tofacilitate the example configuration of FIG. 11;

FIG. 13 is a flow-chart which shows a process that can be implemented toconfigure interrogating probes so as to introduce redundancy tomeasurements;

FIG. 14 is a flow-chart which shows a process that can be implemented toconfigure interrogating probes so as to introduce redundancy tomeasurements;

FIG. 15A is a representation of an embodiment of an interrogating probethat can be generated by an embodiment of the process of FIG. 14, wherethe probe can be configured to interrogate five nucleotides;

FIG. 15B is a representation of an embodiment of an interrogating probewhere the probe can interrogate four nucleotides;

FIG. 15C is another representation of an embodiment of an interrogatingprobe where the probe can interrogate three nucleotides;

FIG. 16 is a flow-chart which shows an embodiment of a process that canbe implemented to generate unique dye assignment schemes such as thoseshown by examples in FIGS. 15A-15C;

FIG. 17 is a block diagram of an embodiment of a system whereperformance of redundancy encoding can be implemented and evaluated;

FIG. 18 is a block diagram of an embodiment of a system whereperformance of redundancy encoding can be implemented and evaluated;

FIGS. 19A-19G is an embodiment of a ligation-based DNA sequencingprocess;

FIGS. 20A-20C are representations of three distinct di-base encodingschemes capable of being utilized with the presently disclosed system;

FIG. 21 is a representation of multiple rounds of ligation-basedsequencing being performed with distinct probe sets;

FIG. 22 is another representation of multiple rounds of ligation-basedsequencing being performed with distinct probe sets;

FIG. 23 is another example of an interrogating probe configured tointerrogate four nucleotides;

FIG. 24 is a representation of a preferred embodiment of ligation-basedsequencing utilizing a di-base specific probe and a 4-base specificprobe;

FIG. 25 is a representation of redundancy incorporated into the encodeddata resulting from the ligation steps depicted in FIG. 24;

FIG. 26 is a representation of an example of a measurement that caninclude redundancy introduced to data via error-correcting code schemessuch as convolutional codes and punctured convolutional codes;

FIG. 27 is an example of how the punctured convolutional coding schemeof FIG. 26 can be implemented to introduce redundancy while measuring alengthy sequence of nucleotides;

FIG. 28 is another representation of an embodiment of the presentlydisclosed encoding scheme being applied to a ligation-based sequencingprocess;

FIG. 29 is a representation of a potential output of an embodiment ofthe presently disclosed system;

FIG. 30 is a representation of an embodiment of encoded data beinginputted into an algorithm, and the resulting output;

FIG. 31 is a representation of an embodiment of data processing by analgorithm of the presently disclosed system;

FIG. 32 is another representation of data processing by an embodiment ofan algorithm of the system;

FIG. 33 is another representation of data processing by an embodiment ofan algorithm of the system;

FIG. 34 is another representation of data processing by an embodiment ofan algorithm of the system;

FIGS. 35A-35E are various representations of steps performed by anembodiment of an algorithm of the system;

FIGS. 36A-36B are various representations of steps performed by anembodiment of an algorithm of the system;

FIGS. 37A-37B are representations of steps performed by an embodiment ofan algorithm of the system;

FIGS. 38A-38C are state graphs resulting from a sequencing processutilizing an embodiment of the presently disclosed 2+4 encoding scheme;

FIG. 39 is a graph which shows examples of the presently disclosedsystem performance that can be expected for variations of the examplecoding scheme of FIG. 28;

FIG. 40 is a block diagram of some embodiments where one or morefeatures of the presently disclosed system can be incorporated into anexisting sequencing system;

FIG. 41 is a flow-chart which shows a process that can be implemented tofacilitate the example configuration of FIG. 40.

FIG. 42 is a schematic diagram of some embodiments of a system for errorcorrecting DNA sample reads using spectral alignment error correction;

FIG. 43 is a representation of an exemplary portion of an alignment ofDNA color call reads, in accordance with some embodiments;

FIG. 44 is a flow-chart showing a method for error correcting DNA samplereads using spectral alignment error correction, in accordance withcertain embodiments; and

FIG. 45 is a schematic diagram of a system of distinct software modulesthat performs a method for error correcting DNA sample reads usingspectral alignment error correction, in accordance with certainembodiments.

These and other aspects, advantages, and novel features of the presentteachings will become apparent upon reading the following detaileddescription and upon reference to the accompanying drawings. In thedrawings, similar elements have similar reference numerals.

DETAILED DESCRIPTION

Systems and methods of determining polymer sequence information, dataanalysis, error detection, error correction, ambiguity detection, andambiguity resolution are provided herein. More specifically, the presentdisclosure provides systems and methods which detect and encode datawhile also introducing redundancy into the encoded data. Redundant datacan be used for error detection and error correction or ambiguitydetection and ambiguity resolution without necessarily having to repeatany data detection and/or measurement steps. This disclosure willgenerally use the phrases “error detection” and “error correction” andthe like, but it will be understood that the disclosure and embodimentsalso relate to identification of ambiguities and resolution ofambiguities.

Error detection and correction can be performed in realtime, on-the-fly,downstream, or at a different time or place from data acquisition. Whilethe presently disclosed encoding schemes and data correction mechanismscan be utilized and tailored towards a wide-range of fields, preferredembodiments are directed for purposes of exemplification herein towardspolynucleotide sequencing applications. In reference-based sequencing,de novo sequencing, and other approaches, the present disclosureprovides tools for acquiring and/or encoding nucleotide-related data ina manner which includes a degree of redundancy. Redundancy can assist inidentifying and correcting errors or uncertainties during decoding ortransformation of the data into sequence information.

The presently disclosed systems and methods can be utilized withvirtually any type of polynucleotide sequencing system or method. Forexample, the encoding and error detection and correction schemes can beused with ligation-based systems, polymerase-based systems,hybridization-based systems, direct or indirect nucleotideidentification systems, pyrosequencing, ion- or pH-based detectionsystems, electronic signature-based systems, etc. In a preferredembodiment, the systems and methods can be utilized with ligation-basedDNA sequencing systems. In particular, the presently disclosed encodingschemes and accuracy enhancements tools can be utilized with systemssuch as the SOLiD DNA Sequencing System (Life Technologies, Carlsbad,Calif.). For purposes of this disclosure, various embodiments areexemplified for teaching purpose in connection with a ligationsequencing approach such as the aforementioned SOLiD system.

As discussed in greater detail below, the SOLiD DNA Sequencing Systemcan be configured to perform multiple ligation rounds offset relative toone another such that each nucleotide is interrogated multiple times.During such interrogations, nucleotide specific signals are generated(e.g., fluorescent signals emitted by various nucleotide specific tags)wherein such signals can be considered the encoded data. That is, insome embodiments, nucleotides can be encoded into color data. Redundantdata can be introduced to the encoded data by interrogating the datawith distinct probe sets. For example, a ligation sequencing process caninclude multiple offset ligation rounds followed by at least oneadditional interrogation event with probes of a distinct probe set. Thatis, the system can perform an additional ligation round(s) configured tointerrogate previously interrogated sites but further be configured toproduce a distinct signal as compared to the signal generated during thefirst interrogation. Taken together, SOLiD's use of multiple ligationoffsets and repeated interrogations with distinct probe sets has beenshown to achieve base-call accuracy of 99.99%. Additional informationregarding the SOLiD ligation sequencing system can be found in U.S.Patent Application Publication No. 2009/0062129, entitled “Reagents,Methods, and Libraries For Gel-Free Bead-Based Sequencing,” the entiretyof which being incorporated herein by reference.

FIG. 1A provides an overview of a preferred embodiment of the presentlydisclosed system 10. As shown, the system 10 can include a RedundantError Correction Code (“ECC”) Encoder 12 configured to encode data intoencoded data. Again, the presently disclosed system can be utilized invarious technologies and/or industries requiring such encryptiontechniques. In a preferred embodiment, the acquired data is related to,derived from, or otherwise representative of a nucleotide sequence.Those skilled in the art will appreciate that such data can be acquiredand encoded by various mechanisms. For example, various sequencingplatforms are known which dispose large amounts of polynucleotidesamples across a substrate and seek to elucidate sequence information byvarious chemical, physical, and/or enzymatic reactions. For example,some platforms bind the samples to bead which are then immobilized on asubstrate, some grow colonies of sample on the substrate, etc. Next, thesamples can be hybridized with or otherwise integrated by a probe having(i) some specificity for one or more nucleotides or sequences ofnucleotides and (ii) a tag (e.g., a fluorophore) wherein excitation ofthat tag can generate a signal indicative of the presence of the target.In some embodiments, the signal(s) generated can be collected as a dataset, as a code, or can be converted into a code or other data form whichcan immediately or at some later time be transformed, interpreted, ordecoded into information regarding base identification. As detailedherein, this data can be introduced to an ECC Decoder 14 configured todetect errors as well as correct such errors without having to repeatany earlier steps.

Errors can be introduced during data acquisition and encoding proceduresin various manners. The explicit cause of such errors is often linked tothe type of data being encoded. Taking ligation-based sequencing as anexample, such systems can produce a clonal DNA sample either on a solidsupport (e.g., a bead) or as a colony directly on a substrate. A mixtureof 4 probes can then be added to the sample wherein the 4 probesinclude, as a general non-limiting example: an A-specific probe, aT-specific probe, a C-specific probe, and a G-specific probe. Asdiscussed in greater detail below, probes specific for variousnucleotide combinations are provided herein which exhibit unexpected andsuperior accuracy results. The four probes are typically labeled with atag capable of being distinguished from the tags of other members of theprobe set. That is, the probes can be fluorescent probes,chemiluminescent probes, etc. In a preferred embodiment, fluorescentprobes are utilized. For example, the four probes can be FAM, Cy3, TXR,and Cy5.

The sample is then excited 4 times in order to preferentially excite oneof the tags during each of the 4 excitation procedures. For example, inthe case of the fluorescent tags, a sample can be irradiated with anexcitation source (e.g., a laser, an arc lap, an LED, etc.) specific orpreferential for FAM, then specific or preferential for Cy3, thenspecific or preferential for TXR, and then specific or preferential forCy5. The desired specificity or preferential collection of data fromtags can also be accomplished in other ways, for example by illuminatingwith one or more excitation sources or wavelengths and filteringemissions from the sample. In an ideal world, the clonal sample would“light-up” only once and would be zero for the remaining 3 excitationsteps. However, systems typically do not behave in an ideal manner anderrors or ambiguity can be introduced. That is, some samples might notbe purely clonal but rather have some amount of contamination. Further,sometimes probes can gather and/or hybridize imperfectly and therebyprovide signals at incorrect locations and times. In view of these kindsof error-inducing situations, a singular or monolithic signal is nottypically generated. Instead, a combination or mix of multiple signalsis produced. In some embodiments, this mix can be considered a mix of 4colors wherein each color is associated with one of the fluorescenttags. In such situations, each signal from each ligation cycle can beconsidered to give a set of 4 color likelihoods as opposed to an exactcolor read. These color likelihoods can form an initial element of theencoded data with some element of error introduced therein.

In view of above, acquiring, storing, and encoding large amounts of datainto a code can increase the probability of encountering an error.Looking again the ligation-based DNA sequencing example, each ligationcycle of each round will generate 4 color-likelihoods for each cycle,which will then be repeated for some number of ligation rounds. Thesecolor-likelihoods will continue to generate the encoded data. However,the presently disclosed system is configured such that successive dataencoding events (e.g., ligation cycles or ligation rounds) not onlyintroduces potential error and/or ambiguities into the code but alsointroduces a degree of redundancy into the code which allows a ECCDecoder 14 to not only decipher the code but to do so in such a mannerwhich allows for error/ambiguity detection and real-timecorrection/resolution. That is, the presently disclosed encoding schemesare capable of allowing the ECC Decoder 14 to not only detect when anerror or ambiguity appears to have occurred but also to determine whatthe correct result should have been, or at least what was the mostprobable correct result. The Decoder 14 is further capable of evaluatingthe various likelihoods/probabilities to therefore determine a mostprobable result without any specific error correction step.

FIG. 1B shows a general block diagram of an embodiment of a sequencingsystem 100 having various components that can facilitate detection andidentification of a sequence of nucleotides in a given sample. In someembodiments, the system 100 can include a detection zone 102 where thesample being sequenced is subjected to various reactions associated withthe sequencing process. Such a detection zone can include, for example,a flow cell, various embodiments of which are disclosed in Assignee'sco-pending U.S. patent application Ser. No. 12/873,190, filed on Aug.31, 2010, entitled “Low-Volume Sequencing System and Method of Use,” theentirety of which being incorporated herein by reference thereto. Aswill be understood by those skilled in the art, various such detectionzone configurations are also possible and within the spirit and scope ofthe present disclosure.

The system 100 can also include an optics component 104 configured toform images of the detection zone, and such images can be formed via adetector 106. The system 100 can also include a processor 108 configuredto control one or more functionalities associated with variouscomponents of the system 100. In certain embodiments, the processor 108can be configured to perform one or more processes as described herein.In certain embodiments, the processor 108 can also be configured tocontrol one or more operations (e.g., detection zone control, opticscontrol, exposure control, detector control, signal acquisition, signalprocessing, analysis of data, etc.) associated with the sequencingsystem 100. Various embodiment of the optics component 104 are disclosedin Assignee's co-pending U.S. patent application Ser. No. 12/873,132,filed on Aug. 31, 2010, entitled “Fast-Indexing Filter Wheel and Methodof Use,” the entirety of which being incorporated herein by referencethereto.

In certain embodiments, the analysis of data may be performed by theprocessor 108. The processor 108 may further be configured to operate inconjunction with one or more other processors. The processor'scomponents may include, but are not limited to, software or hardwarecomponents, modules such as software modules, object-oriented softwarecomponents, class components and task components, processes methods,functions, attributes, procedures, subroutines, segments of programcode, drivers, firmware, microcode, circuitry, data, databases, datastructures, tables, arrays, and variables. Furthermore, the processor108 may output a processed signal or analysis results to other devicesor instrumentation where further processing may take place. The system100 can also include a component 110 configured to detect and correcterrors associated with sequencing processes. In certain embodiments, theerror detection and correction component 110 can be configured toperform one or more of the features as described herein.

The above-described system can be utilized to generate encoded redundantdata from a polynucleotide template. Redundancy allows for errorcorrection by providing multiple and/or distinct measurements of datasuch that these multiple measurements can be compared against oneanother to determine if the measurements are correct. Redundancy canalso require that multiple interrogations are required to determine atrue value of data. In the context of DNA sequencing, redundancy can beintroduced in various manners. For example, taking 3 successivenucleotides, the first and second nucleotides can be interrogated togive a first signal, and the second and third nucleotides can beinterrogated to give a second signal. Thus, multiple interrogationevents can be utilized to interrogate a single data point. Additionally,the system can interrogate the same group of nucleotides with distinctprobes thereby generating distinct signals which are each indicative ofthe same data point (i.e., nucleotide). As detailed below, this approachcan provide a powerful accuracy enhancing sequencing platform whencombined with the use of carefully selected and constructed probe sets.

FIG. 2A shows a block diagram of a configuration 120 where redundancycan be introduced to a block of data 122 having a length K. The block ofdata 122 can represent, for example, a sequence of nucleotides in a DNAsample strand being analyzed. In such a context, the length K canrepresent number of bases (if a single strand) or base pairs (if adouble strand) in the sample strand.

For the purpose of description, a unit (such as a base or base pair) inthe length K may be referred to as a “digit”, “symbol”, or other term.Such terms are commonly associated with error correcting code (“ECC”)terminology, however, as used herein, such terms are not necessarilylimited by previously-cited definitions. Thus, as used herein, theseterms may reflect multiple values (e.g., those values associated withthe base sequences A, C, G, T). Therefore, the data 122 can be referredto as a K-digit data, and/or K-symbol data interchangeably withoutlimiting or departing from the scope of the present disclosure. In thecontext of a sequence of bases, each symbol can represent one of fourbases A, C, G, T.

As shown in FIG. 2A, the K-symbol data 122 is shown to be encoded by anencoding component 124 so as to yield a code 126 having a length M.Similar to data length K, a unit in the length M may also be referred toas a symbol or some other term commonly associated with ECC terminology.Thus, the code 124 can be referred to as an M-symbol code. In thecontext of certain fluorescence-based sequencing processes, each symbolcan represent one of a number of fluorescence colors.

In general, redundancy introduced in the foregoing manner can result ininformation content of the code 126 being greater than informationcontent of the data 122. Thus, redundancy can be considered to beintroduced to the data 122 if N^(M)>L^(K), where M (referred to as thelength in FIG. 2A) represents the number of detectable characteristics(such as fluorescence colors) with each characteristic being one of Ndiscrete values, and K (referred to as the length in FIG. 2A) representsthe number of unknown characteristics with each characteristic being oneof L discrete values. In the context of a sequence of bases (where eachbase can be one of four bases A, C, G, and T), L=4. In the context of afluorescence-based detection context, an example 4-color configurationcorresponds to N=4. As described herein, various configurationsassociated with sequencing operations can be selected so as to yield theredundancy condition N^(M)>L^(K).

In certain embodiments, coded information having redundancy can bedecoded so as to facilitate detection of sequencing errors andcorrection of such detected errors and/or detection and/or resolution ofambiguity (i.e., there does not necessarily include an error correctionbut rather a resolution of some uncertainty or ambiguity). FIG. 2B showsa block diagram of a configuration 130 where a code 132 havingredundancy (e.g., generated by the encoder 124 of FIG. 2A) can bedecoded by a decoding component 134. Such an operation can yield data136 extracted from the code 132, as well as information (depicted ascomponent 138) that facilitates detection and correction of an errorthat may have occurred between data 122 “before errors” and data 136“after errors.” It will be appreciated that “error detection” and “errorcorrection” may be separated into discrete tasks or combined in a jointor serial manner. In various embodiments the decoder may be implementedto determine the Data that, if encoded using the Encoder, may yield aCode that is similar or most similar to the observed “Code with errors”.As will be appreciated, such an approach may be used to achieve “errorcorrection” without an explicit “error detection” step or routinerecognizing that in various instances actual errors may take place in“Code”-space and not necessarily in “Data”-space.

FIG. 2C provides another representation of the presently disclosederror-correction scheme wherein the introduction or redundancy andencoding selection are illustrated as distinct processes. That is, anaccuracy-enhancing polynucleotide encoding/detection scheme 151 of thepresent disclosure can be considered to include a RedundancyIntroduction Process 153 which can include, for example, transformingdata into encoded redundant data where, for example N^(M)>L^(K). Thescheme 151 can also include Encoding Selection 155 which can include dyeassignment to probes, etc. Those skilled in the art will appreciate thatthese distinct “Processes” are shown as such merely for explanatorypurposes and various steps in the presently disclosed scheme can beconsidered to fall within either or both such blocks 153, 155.

Redundancy can be introduced and incorporated with data in variousmanners. For example, FIG. 3 shows a sequencing situation where KAsymbols of a polynucleotide template strand 142 are being interrogatedby a probe 140. As shown, the probe 140 can be configured to include KAsymbols that complement the KA symbols of the template strand 142. Thus,example nucleotide sequence GTC of the template 142 can hybridize withthe complementary sequence CAG of the probe 140.

The example probe 140 can have one or more additional symbols so as toyield S symbols. In some embodiments, as shown in FIG. 3 and describedin detail below, the probe 140 can also include additional bases,represented as “Z”, which can be cleaved during processing. In certainembodiments, such S symbols can allow S rounds of interrogation whereeach interrogation round is performed at a selected offset of the probe140 relative to the template strand 142. For example, a probe having theCAG sequence is shown to hybridize to the GTC sequence of the templatestrand in Step 1. In Step 2, another probe having an AGA sequence isshown to hybridize to a TCT sequence on the template strand, where theTCT sequence is offset from the GTC sequence of Step 1 by onenucleotide. Such offset interrogation can be performed at least S times.

In FIG. 3, the example probe 140 is shown to have detectable labels 144indicated as “p” in Step 1 and “q” in Step 2, etc. In some embodiments,each of the labels 144 (“p,” “q,” etc.) can belong to a same group andhave a unique assignment of, for example, a dye color based on theprobe's nucleotide sequence. For example, the probe having the CAGsequence (Step 1) can be assigned with one of a number of dyes in aunique manner. Similarly, the probe having the AGA sequence (Step 2) canbe assigned with one of the number of dyes in a unique manner.

In some embodiments, the probes 140 can also belong to two or moredifferent groups, where each group has a unique dye assignment schemefor its labels. For example, a first group of probes can have a uniqueassignment of dye color for the probe sequence CAG of Step 1; and asecond group of probes can have another unique assignment of dye colorfor the same probe sequence CAG of Step 1. As described herein, suchdifferent groups of probes can allow contribution of redundancy incertain situations. For example, if probes belonging to one group areutilized, redundancy can be represented by the number of rounds ofoffset interrogations (e.g., in FIG. 3, S rounds); and interrogation ofa given offset more than once is simply repetitive and does notcontribute to redundancy. However, if a given offset is interrogatedagain by a probe from a different group, then such an interrogation cancontribute additional useful information for the purpose of redundancy.

Considered in another manner, the sequencing procedure may be likened toa pair of nested FOR-loops. The outer loop may be referred to as PrimerRounds, and the inner loop a Cycle. In the first primer round, firstcycle, a probe (e.g., probe 140) may attach to nucleotides 1 through 5.In first primer round, second cycle (following cleavage of the “ZZZ”nucleotide sequence of probe 140), a probe may attach to nucleotides 6through 10. Subsequent cycles within the same primer rounds may examinepositions 11-15, 16-20, and so forth, until a reset is performed and anew primer round started (e.g., Step 2 in FIG. 3). At the second primerround, a probe may be attached at positions 2-6 at the first cycle, and7-11, 12-16, 7-21, etc, at subsequent cycles. Similarly, Primer round 3may examine positions 3-7, 8-12, etc, and Primer round 4 examinepositions 4-8, 9-13, etc, with Primer round 5 examining positions 5-9,10-14, etc. Referring to FIG. 3, the variable “X” can be considered torepresent the number of Primer Rounds which may or may not equal “S”.That is, as described in greater detail below, the total number ofPrimer Rounds can include at least one additional round performed with adistinct probe set.

In FIG. 3, X rounds of interrogation (which each include any number of“cycles”) are depicted as yielding a code 150 having a length of M. Asdescribed herein, redundancy can be considered to be satisfied ifN^(M)>L^(K). In the context of sequencing operations such as that shownin FIG. 3, the quantity M (number of detectable characteristics) can berepresented as M=K*(X/S), where S represents the number of offsets(e.g., 5 in FIG. 3) and X represents the number of unique rounds (e.g.,5 in FIG. 3 due to 5 offsets). As described herein, the quantity X caninclude the number of offset measurements, as well as measurement(s) ofa same offset using probes from different group(s). Thus, it will beappreciated that the number of primer rounds X need not necessarily beequal to the number of unique primer offsets S. As such, the value of Xand S may be independently determined.

In FIG. 3, L=4 (corresponding to 4 types of bases); thus, redundancy canbe achieved if N^(K)*^((X/S))>4^(K). Values for N, X, and/or S can beselected so as to meet the redundancy requirement. For example, for asystem that uses 4 types of dyes (N=4), redundancy can be achieved ifK*(X/S)>K, or X>S. For a system that utilizes S offsets, the redundancyrequirement of X>S can be achieved by, for example, performing S offsetmeasurements plus one or more measurements (at one or more of the sameoffset values) using probes belonging to different group(s). Examples ofsuch redundancy-introducing methods are described herein in greaterdetail.

In the context of L=4, the redundancy requirement ofN^(K)*^((X/S))>4^(K) can also be achieved by selecting an appropriatevalue for the number of dye types. For example, and as shown in aconfiguration 160 of FIG. 4A, if 5 types of dyes 164 (N=5) are providedfor probes 162, the redundancy requirement becomes 5^(K)*^((X/S))>4^(K).Thus, even if only S rounds of unique measurements (X=S) are made, theredundancy requirement is met, since 5^(K)*(S/S)>4^(K), or 5^(K)>4^(K).

In certain embodiments, an effective value for N can be made relativelylarge by providing more than one dye per probe. Such an increase in Ncan allow one or more of the other parameters to be adjusted (e.g.,reduced) accordingly, and yet satisfy the redundancy requirement.

For example, FIG. 4B shows a configuration 170 of a probe 172 having twodyes d1 and d2 (174 a, 174 b). If each of the two dyes can be one offour types, the value for N can be represented as 2⁴−1 possiblecombinations of the two dyes unique combinations (for example, 11, 12,13, 14, 22, 23, 24, 33, 34, 44 where 12, for example, may be opticallyindistinguishable from 21). With such a relatively large value for N,the quantity X (number of unique measurements) can be less than S(number of offsets) and yet satisfy the redundancy conditionN^(K)*^((X/S))>L^(K), which can also be expressed as K*(X/S)*log N>K*logL, or (X/S)*log N>log L. Thus, for example, if X=3 for the S=5configuration, the redundancy requirement is met, since(3/5)*log(15)>log(4), or 0.71>0.60.

In another embodiment, a sequencing reaction may be performed on acollection of substantially identical or identical polynucleotide clonesin parallel. In this case where there may be a relatively large numberof clones, a relatively large number of identical probes anneal to themduring one cycle, and it may be the joint fluorescence of these probesthat may be being measured by the optics. In this context, it may besuggested that a particular probe sequence, “ACTGC”, and dyes a, b, c,d, could simultaneously have a probe ACTGC-a and ACTGC-b in the mixture(as distinguished from multiple probe sets here both probes may be in asingle set). With this, if a complementary polynucleotide is beingsequenced, probe ACTGC-a may bind to roughly a half of the clones andACTGC-b to the other half. It may then be observed that both colors aand b are found with half the intensity. For example, for an arbitraryprobe sequence, one may select a single probe type with one of the fourdyes, or a mix of two probes with different dyes (6 possibilities), or amix of three probes (4 possibilities) or a mix of four probes with eachdye—15 options altogether.

Contribution to redundancy can also be made by interrogating same Ksymbols of bases more than once with different probes. FIG. 5 shows anexample of such interrogations. In a first interrogation 180 a, anexample sequence GTC is shown to be interrogated by a first probe 182having a dye “p” 184. The dye 184 can be one of a number of types (e.g.,one of four types of dyes). In a second interrogation 180 b, the samesequence GTC is shown to be interrogated by a second probe 186 having adye “q” 188. The dye 188 can be one of a number of types (e.g., one offour types of dyes).

In certain embodiments, each of the dyes 184, 188 can be selected fromfour types of dyes (N=4). Thus, the probes 182, 186 may or may not havesame dye. In certain embodiments, difference between the probes 182, 186can be achieved due to a difference in how such dyes are assigned, evenif the probes end up with same dye.

In certain embodiments, the first probe 182 can be part of a first groupof probes, where each probe in the group is assigned a dye based on anassignment scheme unique to the first group. Similarly, the second probe186 can be part of a second group of probes, where each probe in thegroup is assigned a dye based on an assignment scheme unique to thesecond group. Examples of such unique assignments are described hereinin greater detail.

As described in reference to FIGS. 3-5, redundancy can be introduced byone or more different manners. For example, redundancy can be introducedby configuring the probes in certain manners. Redundancy can also beintroduced by interrogating the template strand in certain manners.

FIG. 6 shows a process 190 that can be implemented to introduceredundancy to measurements. In a process block 192, a number ofnucleotides to be interrogated can be determined. In certainembodiments, such number of nucleotides can be interrogatedsubstantially simultaneously. In a process block 194, probes and/ormanner of interrogations can be configured so as to yield a code thatincludes data representative of the number of nucleotides, and at leastsome redundancy that facilitates error detection and correction.

FIGS. 7-12 provide additional examples of configurations and processesthat can be implemented to introduce redundancy as described by way ofexamples in reference to FIGS. 4 and 5. More particularly, FIG. 7 showsa more specific example configuration 200 of the redundancy-introducingconfiguration of FIG. 4A. As shown, a template strand 204 is probed byprobes 202 in five offset rounds. In Round 1, a probe having a sequenceCAGAT is shown to hybridize to a 5-symbol sequence GTCTA on the templatestrand 204. In Round 2 (not shown), another probe having a sequenceAGATT hybridizes to a shifted 5-symbol sequence TCTAA on the template204. Additional rounds are performed on progressively shifted 5-symbolsequences, and in Round 5, a probe having a sequence complementary tothe 5-symbol shifted template sequence is shown to be hybridized.

As shown in the example configuration of FIG. 7, each of the dyesassigned to the probes can be one of five types (d=1, 2, 3, 4, 5) suchthat N=5. As described herein, use of five types of dyes can satisfy theredundancy requirement even when each offset template sequence isinterrogated only once.

FIG. 8 shows a process 210 that can be implemented to facilitate theexample configuration 200 of FIG. 7. In a process block 212, a number oftypes of dyes greater than the number of nucleotide types can beprovided. In FIG. 7, the number of nucleotide types is four (L=4), andthe number of types of dyes is 5 (N=5). In a process block 214, dyes canbe assigned to probes having different K-symbol sequences. In a processblock 216, a template strand can be interrogated at least P times, whereP represents a number of offsets of the probe relative to the template.In FIG. 7, the template is interrogated five times at five differentoffsets.

In some respects, the aforementioned multi-dye example bearssimilarities to the dye-combinations in previous examples, and thus theformula N^(K)*^((X/S))>L^(K) applies. Another approach to analysisinstead of introducing P, says that the number of primer rounds X>S*(logL)/(log N). With S=5, N=5, L=4, this means X>4.3, which is satisfied forX=5. In various embodiments, this reflects an example of a special casein that with X<=5 one probe set may be used (multiple probe sets may beused when primer rounds reuse the same offset, and there are S=5possible offsets). Additionally, if even more dyes are used, for exampleN=7, then X>5*log 4/log 7, i.e., X>3.5, and this may suggest that X=4 isredundant. Consequently, it will be appreciated that S*(log L)/(log N)may reflect the lower bound on X.

FIG. 9 shows a more specific example configuration 220 of theredundancy-introducing configuration of FIG. 4B. As shown, a templatestrand 224 is shown to be probed by probes 222 in three offset rounds.In Round 1, a probe having a sequence CAGAT is shown to hybridize to a5-symbol sequence GTCTA on the template strand 224. In Round 2, anotherprobe having a sequence AGATT is shown to hybridize to a shifted5-symbol sequence TCTAA on the template 224. In Round 3, another probehaving a sequence GATTG is shown to hybridize to a shifted 5-symbolsequence CTAAC on the template 224.

As shown in the example configuration of FIG. 9, each of the probes isshown to have two dyes d1 and d2; and each of the two dyes d1 and d2 canbe one of four types (d=1, 2, 3, 4) such that combined value for N is 15as described in reference to FIG. 4B. As described herein, use of twodyes per probe can satisfy the redundancy requirement even when aK-symbol template sequence is interrogated at a number of offsets thatis less than the probe's offset capacity S (e.g., S=5 in the exampleconfiguration 220 of FIG. 9).

FIG. 10 shows a process 230 that can be implemented to facilitate theexample configuration 220 of FIG. 9. In a process block 232, a number oftypes of dyes can be provided. In certain embodiments, the number oftypes of dyes can be at least as much as the number of types ofnucleotides (L=4); thus, in FIG. 9, the number of types of dyes can befour. In a process block 234, more than one dye can be assigned to eachof the probes having different K-symbol sequences. In FIG. 9, two dyes(each being one of four types) can be assigned to each of the probes soas to yield a combined value of 15 for the quantity N. In a processblock 236, a template strand can be interrogated at least a number oftimes that is less than the probe's offset capacity. In FIG. 9, theprobe's offset capacity S=5, and the template strand is interrogatedonly three times at different offsets.

FIG. 11 shows a more specific example configuration 240 of theredundancy-introducing configuration of FIG. 5. As shown, a templatestrand 244 is shown to be probed by probes belonging to differentgroups. For the purpose of description of FIG. 11, probes in a firstgroup are indicated as “p,” and probes in a second group are indicatedas “q.” In Round 1, a probe “p” having a sequence CAGAT is shown tohybridize to a 5-symbol sequence GTCTA on the template strand 244. InRound 2, a probe “q” having a sequence CAGAT is shown to hybridize tothe same 5-symbol sequence GTCTA on the template strand 244. In Round 3,another probe “q” having a sequence AGATT is shown to hybridize to ashifted 5-symbol sequence TCTAA on the template 244. Additional roundsare performed on progressively shifted 5-symbol sequences, and in Round5, a probe “q” having a sequence complementary to the 5-symbol shiftedtemplate sequence is shown to be hybridized.

In the example configuration of FIG. 11, six rounds of interrogation areperformed by five offset rounds with probes “q” and one round with probe“p.” It will be understood that other combinations of numbers ofround(s) for each group of probes are also possible. Further, the offsetlocation of the two-round measurements can also be different than theexample shown in FIG. 11.

FIG. 12 shows a process 250 that can be implemented to facilitate theexample configuration 240 of FIG. 11. In a process block 252, a numberof types of dyes can be provided. In FIG. 11, the number of types ofdyes is 4 (N=4) and same as the number of types of nucleotides (L=4). Ina process block 254, dyes can be assigned to probes in two groups “p”and “q.” Examples of such dye assignments are described herein ingreater detail. In a process block 256, a template strand can beinterrogated at least a number of times that is more than a number ofoffsets of the probe relative to the template. Such interrogation(s)beyond the probe's offset capacity can be provided by one or moreadditional groups of probes. In FIG. 12, there are two groups of probes(“p” and “q”), and probes of one group (“q”) provide five rounds, andthe sixth interrogation is provided by “p” probe.

The presently disclosed encoding schemes utilize probes having uniquelyassigned dyes which complement the redundancy to provide the ability todetect and correct errors. FIGS. 13-15 show non-limiting examples of howsuch unique dye assignments can be achieved. Although dyes are used asexamples of detectable components, it will be understood that othertypes of detectable components having different detection properties canalso be used. For example, detectable components can be based onemissions other than fluorescence. In another example, detectablecomponents can be based on emission of electromagnetic radiation otherthan those generally in the visible range.

FIG. 13 shows a process 260 that can be implemented to assign dyes toprobes. In a process block 262, N types of dyes can be provided. In aprocess block 264, probes having KB-symbol nucleotide sequences can beprovided. In a process block 266, the process 260 provides one of the Ntypes of dyes to each KB-symbol sequence based on a selected assignmentscheme.

In certain embodiments, the process 260 can generate one group ofprobes, where each of the probes in the group undergo dye-assignmentbased on the same selected assignment scheme. In situations where one ormore groups of probes are desired, a process similar to the process 260can be performed using different assignment scheme(s).

FIG. 14 shows a process 270 that can be implemented as a more specificexample of the process 260 of FIG. 13. More particularly, the process270 can be configured to generate probes where four types of dyes areutilized. Thus, in a process block 272, four types of dyes can beprovided. In a process block 274, probes having KC-symbol nucleotidesequences can be provided. In a process block 276, a unique generatorvector corresponding to KC-symbols can be selected. Steps of processblocks 272, 274, and 276 can be performed once while the remaining steps(process blocks 278, 280, 282 can be performed separately for eachprobe). That is, in a process block 278, a multiplication operationbetween the KC-symbol sequence of a given probe and the selectedgenerator vector can be performed. In certain embodiments, the generatorvector can be based on a known Galois field GF(4). In a process block280, a summation operation can be performed on the product resultingfrom the multiplication operation of the process block 278. In certainembodiments, such summation can be based on the Galois field GF(4). In aprocess block 282, one of the four types of dye can be selected based onthe summation value.

FIGS. 15A-15C show specific but non-limiting examples of dye assignmentsthat can be achieved via the process 270 of FIG. 14. In FIG. 15A, a dyeassignment configuration is shown for a probe 290 having 5-symbols ofnucleotides 292. For the purpose of description, the probe 290 isdepicted as having a 3′ end and a 5′ end, with a dye 294 disposed on the5′ end. Further, the probe 290 is depicted as being an octamer with the5-symbols of nucleotides and three unused symbols of universal bases(depicted as “z”). Such a configuration allows use of the probe 290 insequencing systems such as SOLiD. It will be understood, however, thatvarious features such as 3′ and 5′ ends, particular position of the dye,octamer, and/or use of universal bases are not necessarily requirementsfor the purpose of encoding selection.

In FIG. 15A, an example 5-symbol sequence of AGATC is shown to betransformed to numerical representation 02031 based on map 300, where Ais represented by 0, C by 1, G by 2, and T by 3. The 5-symbol numericalrepresentation of the 5-symbol sequence is shown to be multiplied with aselected generator vector 310. The generator vector 310 is shown asg_(i)=(1, 3, 1, 1, 2). Selection of such generator vectors is describedherein in greater detail.

In certain embodiments, the multiplication of the 5-symbol numericalrepresentation 02031 and the generator vector 13112 can be performedbased on a rule such as a Galois field GF(4) multiplication table 302.Thus, multiplication of the first symbols (0 and 1) yields 0,multiplication of the second symbol (2 and 3) yields 1, and so on, so asto yield a product 01032.

In certain embodiments, members of the product 01032 can be summed basedon a rule such as a Galois field GF(4) addition table 304. Thus,addition of symbols 1 and 2 (0 and 1) yields a sum of 1, and addition ofthat sum with symbol 3 (0) yields a sum of 1. Continuing, addition of 1with symbol 4 (3) yields a sum of 2, and addition of 2 with symbol 5 (2)yields a sum of 0. Thus, the sum of the product 01032 is shown to be 0.

In certain embodiments, the resulting sum can be assigned to one of thefour dye types. In the example shown in FIG. 15A, the four dye types areshown as four colors blue (“b”), green (“g”), yellow (“y”), and red(“r”). Thus, based on a color assignment map 306 (0 assigned to b, 1 tog, 2 to y, and 3 to r), the sum 0 is shown to result in a blue dye (294)being assigned to the probe having a sequence of AGATC.

FIG. 15B shows a similar dye color assignment scheme for a probe 320having a 4-symbol sequence GACG based on an example generator vector322. Base-to-number map, multiplication rule, summation rule, andnumber-to-color map can be similar to that described in reference toFIG. 15A.

In FIG. 15B, the fifth symbol is unused for the purpose of colorassignment to the 4-symbol sequence; and is shown to be occupied by adegenerate base (depicted as “n”). Introduction of such a base into aprobe such as the example octamer probe of FIG. 15B is generally knownin the art.

In FIG. 15B, the example 4-symbol sequence GACG yields a summation valueof 1. Thus based on the color assignment table (306 in FIG. 15A), agreen dye is assigned to the probe 320.

FIG. 15C shows a similar dye color assignment scheme for a probe 330having a 3-symbol sequence TCA based on an example generator vector 332.Base-to-number map, multiplication rule, summation rule, andnumber-to-color map can be similar to that described in reference toFIG. 15A. The example 3-symbol sequence TCA yields a summation value of2; thus, a yellow dye is assigned to the probe 330.

In FIGS. 15A-15C, example generator vectors g_(i)=(1, 3, 1, 1, 2),g_(i)=(1, 3, 2, 3), and g_(i)=(1, 1, 2) are used to multiply thenumerical representations of 5-symbol, 4-symbol, and 3-symbol probesequences. In certain embodiments, such generator vectors can beselected based on one or more factors that contribute to the robustnessof the resulting encoding. For example, Hamming distance is a parameterthat represents a distribution of encoded codes. If the encoding processresults in the codes being “crowded” together (small Hamming distance),then decoding of such codes may be more susceptible to additionalerrors. Thus, having a relatively large Hamming distance among the codescan be beneficial.

FIG. 16 shows a process 340 that can be implemented to select one ormore generator vectors. In a process block 342, a type of coding schemecan be selected. Examples of such coding schemes are described herein ingreater detail. In a process block 344, one or more generator vectorscan be obtained based at least in part on relatively large Hammingdistance among the codes generated by the generator vector(s).

In certain embodiments, performance of encoding operation and/orredundancy-introducing operation may not be readily apparent whileselecting the generator vector(s). Thus, as shown in FIG. 17, a feedbacksystem 350 can be implemented to facilitate evaluation of one or moreperformance parameters associated with, for example, generatorvector(s).

In certain embodiments, the feedback system 350 can include a basesequence generator 352 configured to generate a sequence of bases to bemeasured. Such a sequence can be provided to an encoder 354 configuredto generate codes having redundancy. Such encoding can include, forexample, dye color assignments based on given generator vector(s) and/orredundancy-introductions.

Codes resulting from the encoder 354 can be provided to a modelingcomponent 356. Such a component can be configured to simulate, forexample, signal detection and processing, and dye color determination.Such simulated measurements can be provided to a decoder 358 where themeasured colors can be decoded.

As shown in FIG. 17, the decoded output data can be compared to theinput data (from the base sequence generator 352) by a performanceevaluation component 360. Such a component can evaluate how accuratelythe input data was preserved in the output. If any error is detected bythe comparison, the performance evaluation component 360 can alsoevaluate how well such errors can be corrected.

FIG. 18 shows a more specific example configuration 370 of the feedbacksystem 350 of FIG. 17. For example, the base sequence generator 352 ofthe system 350 can be a random or pseudo-random base sequence generator372. In another example, the encoder 354 can be a convolutional encoder376 known in the art; and the decoder 358 can be a Viterbi decoder knownin the art. In yet another example, the performance evaluation component360 can be a base mis-call counter 380.

As is known, convolutional code is a type of a linear coding schemewhere mapping occurs for sliding windows of symbols. In the context ofcoding a sequence of bases, such a sliding window can be a K-symbolsequence in a template strand. For convolutional coding (and assuming asituation where four dye colors are being used), two generator vectorsg₁ and g₂ and the resulting groups of probes can facilitate sufficientnumber of unique measurements to introduce redundancy. As describedherein, one group of probes (having one of four colors) can provide S(e.g., 5) unique measurements via S offsets; and one or more additionalunique measurements can be provided by a second group of probes (up toanother S unique measurements).

As is also known, limiting the search for desirable generator vectors totwo (g₁ and g₂) generally limits the number of unique codes torelatively small numbers. In the context of the Galois field GF(4)configuration (FIGS. 15A-15C), there are 256 unique 256 3-symbolgenerator vectors, 4,096 4-symbol generator vectors, and 65,536 5-symbolgenerator vectors.

Based on the foregoing, two generator vectors g₁ and g₂ having desirableperformance attributes can be identified. As described in reference toFIG. 16, Hamming distance is a factor that can be important. However,other factors can be considered, since a number of generator vectors canyield similar large Hamming distances.

Table 1 lists example generator vectors that have been identified asproviding desirable performance attributes (such as large Hammingdistance) for different example measurement configurations:

TABLE 1 Number of Number of Hamming symbols (K) measurements g₁ g₂distance 3 6 (1, 2, 1) (1, 3, 1) 3 4 6 (1, 1, 1, 2) (1, 2, 0, 1) 4 5 6(1, 1, 2, 2, 1) (1, 2, 0, 0, 1) 4 5 7 (1, 2, 3, 2, 1) (1, 0, 3, 3, 3) 5As listed in Table 1, and in the context of 5-symbol configuration,performing seven rounds of measurements can increase the resultingHamming distance (5) from the six-round case (Hamming distance=4). Asdescribed herein, Hamming distance can be an important factor, but notnecessarily the only factor to be considered overall. For example, ifthe additional round (seventh round) is costly and time consuming, theincreased Hamming distance provided by the extra round may not bebeneficial.

For the example measurement configurations listed in Table 1, the codedprobes have offset capacity of five symbols. Thus, five uniquemeasurements can be achieved using one of the two generator vectors g₁and g₂. To provide redundancy (when using one dye per probe and fourcolors), one or more additional unique measurements can be made usingthe other generator vector. For example, and as described in referenceto FIG. 11, the other generator vector can be used for one additionalmeasurement. In a preferred embodiment, when selecting such distinctprobe sets to be used together, the sets can be configured relative toone another to provide optimized results (e.g., accuracy). That is, thesets are preferably not devised in isolation but rather in view of oneanother such that the multiple interrogations of the data by thedistinct probe sets can minimize ambiguity during decoding of the data.

Probe sets can also be carefully and specifically constructed so as tooptimize the particular data encoding process for which the presentlydisclosed systems and methods are utilized. For example, specific probesets are disclosed herein which are optimized for use withligation-based DNA sequencing, in particular, for use with the SOLiD DNAsequencing system. While the following discussion will focus on suchsystems, those skilled in the art will appreciate that the presentlydisclosed teachings can be applied to optimize probe selection andencoding schemes for various other types of DNA sequencing systems(e.g., other ligation-based systems, polymerase-based systems, etc.).

FIGS. 19A-19G provides a general overview of an embodiment ofligation-based sequencing chemistry utilized with the SOLiD system. FIG.19A shows Primer Round which includes initialization using a firstinitializing oligonucleotide that is hybridized to an adapter sequence(referred to above as a binding region) in the template to provide anextendable duplex. As detailed below, in some embodiments the last baseof the adaptor sequence is a known value thereby providing a startingpoint for later base determination which takes place after all ligationcycles and rounds are complete. FIG. 19B shows excitation of the tags(e.g., fluorophores) thereby producing signal (e.g., colors) to bedetected by the optical system. FIG. 19C shows a step wherein thosestrands which did not participate in Primer Round 1 are “capped” therebypreventing them from participating in future ligation cycles. FIG. 19Cshows a cleavage step where the tag is removed thereby preparing thestrand for future ligation rounds.

FIG. 19E shows several ligation cycles of nucleotide identification inwhich every various nucleotides are interrogated in order to generatesignal(s). Following a desired number of cycles, the extended strand,including the first initializing oligonucleotide, is removed and asecond oligonucleotide that binds to a different portion of the bindingregion from that at which the first initializing oligonucleotide bound,is hybridized to the template. FIG. 19F shows a second sequencingreaction, the Primer Reset, in which initialization is performed with asecond initializing oligonucleotide, followed by several cycles ofnucleotide interrogation. FIG. 19G then shows extension from the secondinitializing oligonucleotide allows interrogation of nucleotides in adifferent “frame” from the nucleotides interrogated in the firstsequencing reaction.

As summarized above, the SOLID system enables massively parallelsequencing of clonally amplified DNA fragments linked to beads. Asshown, this sequencing methodology is based on sequential ligationoligonucleotide probes labeled with one of four fluorescent dyes,Whereby each probe can assay up to 5 base positions as a time. Everywindow of five consecutive bases of DNA fragment is interrogated atleast once (as controlled via probe cleaving, primer hybridization, andprimer resets), and some windows are interrogated twice. In the latercase, the first and second inspections are performed using differentlylabeled probe sets, carefully designed to form a redundant errorcorrecting code. The set of all dye color measurements, each carryinginformation about multiple bases, is then used by specialized decodingalgorithms to establish the most likely base sequence (before and aftermapping), even in the presence of measurement errors.

As such, in some embodiments, the above-described ligation rounds areperformed with distinct sets of probes. That is, an initial series ofligation rounds are performed offset from another. Further, theseligation rounds can be performed with specifically constructed probessuch that each nucleotide is interrogated at least twice. In a preferredembodiment, the initial ligation rounds utilize di-base specific probes.That is, the probes are specific for 2 adjacent nucleotides. Thus,during the initial series of offset ligation rounds, each nucleotidewill be interrogated twice: first, as the first nucleotide of a pair,and second, as the second nucleotide of a pair. These interrogationswill provide two signals. The first signal generated by the firstinterrogation event and the second signal generated by the secondinterrogation event. Each of these two signals is required to determinethe identity of a single base. That is, when considering 3 consecutivenucleotides, the identity of the middle nucleotide can only bedetermined by evaluating the first signal generated when nucleotides 1and 2 are interrogated in view of the second signal generated whennucleotides 2 and 3 are interrogated.

The system can also employ at least one additional round of ligationwhich interrogates the same data as was interrogated during the initialligation rounds. However, this additional ligation round can utilizeprobes of a distinct probe set thereby providing two signals for thesame data thereby providing redundant encoded data.

FIG. 20A-FIG. 20C provides three examples of di-base specific probescapable of being used in the above-described DNA ligation-basedsequencing. These encoding schemes, similar to the 5-base probesdiscussed above, are generated by use of distinct generator vectors inaddition to Galois field GF(4) look-up table 307, multiplication table309, and summation table 311. That is, FIG. 20A provides a first 2-baseencoding scheme 301 utilizing generator vector G=(1 1), and theColor=Base1+Base2. FIG. 20B provides another embodiment of a 2-baseencoding scheme 303 where the generator vector is represented as G=(1 2)and the Color=Base1+2×Base2. FIG. 20C provides yet another embodiment ofa 2-base encoding scheme 305 where the generator vector is representedas G=(1 3) and the Color=Base1+3×Base2. For each, “0” represents Blue,“1” represents Green, “2” represents Yellow, and “3” represents Red.Those skilled in the art will appreciate that various other such schemesare within the spirit and scope of the present disclosure.

FIG. 21 provides an example of ligation-based DNA sequencing utilizingthe probe sets prepared in accordance with FIG. 20A and FIG. 20B. Thatis, as shown, the first 5 ligation rounds, designated as 313, areperformed with probe set 1 which includes di-base specific probesprepared in accordance with the teaching of FIG. 20A (probe setidentified as (1, 1) in accordance with the generator vector). Note thateach of these rounds is performed at a 1-base offset relative to afollowing round such that each base is interrogated twice. In reviewingFIG. 21 (as well as FIGS. 22 and 24), note that symbol “0” correspondsto Blue, symbol “1” corresponds to Green, symbol “2” corresponds toYellow, and symbol “3” corresponding to Red.

A 6^(th) ligation round, designated as 315, is performed with probes ofprobe set 2 which includes probes prepared in accordance with theteachings of FIG. 20B (probe set 2 identified as (1, 2)). Note that thissecond probe set provides a second interrogation of nucleotides whichhave already been interrogated by the first probe set. However, makingreference to FIGS. 20A and 20B, probe set 2 will give a different signal(e.g., color) for the same interrogation event as compared to probeset 1. In a preferred embodiment, probe set 1 and probe set 2 areselected in view of one another so as to provide optimized systemaccuracy.

FIG. 22 provides another example which illustrates that variouscombinations of first and second probe sets are within the spirit andscope of the present disclosure. That is, FIG. 22 provides an examplewhere ligation rounds 1-5 are performed with a 5-base specific probe,and ligation round 6 is performed with a distinct set of 5-base specificprobes. In accordance with the teachings of FIG. 15A, such distinct5-base specific probes can be prepared by careful selection andoptimization of generator vectors. Those skilled in the art willappreciate that any such combination of distinct probe sets, use of morethan two distinct probe sets, etc. are within the spirit and scope ofthe present disclosure.

In a preferred embodiment, the presently disclosed system can beemployed during DNA sequencing utilizing 5 offset ligation rounds withthe di-base specific probe prepared in accordance with the teachings ofFIG. 20A (probe set (1, 1)) and a 6^(th) ligation round utilizing a4-base specific probe set. FIG. 23 provides a preferred embodiment ofsuch a 4-base specific probe. Like above, various generator vectors canbe utilized in this embodiment. In a preferred embodiment, the generatorvector is G=(1 3 0 3) which is utilized with Galois field GF(4) look-uptable 319, multiplication table 321, and summation table 323. As shown,the probe sequence of ACAGT can correspond to a value of 2 whichcorresponds to a specific tag or color. Note that probe set (1, 3, 0, 3)has a special property that the color does not depend on base position 3because of the 0 at the position 3

FIG. 24 provides a representation of this preferred embodiment utilizingprobe set 1=(1, 1) and probe set 2=(1, 3, 0, 3). This encoding schemecan be referred to as (“2+4” encoding). Table 2 provides the preferredlabeling scheme for 2+4 encoding:

Sequences detected by labeled probes Probe label FAM (0) Cy3 (1) TXR (2)Cy5 (3) Probe set 1 AA*** AC*** AG*** AT*** CC*** CA*** GA*** TA***GG*** GT*** CT*** CG*** TT*** TG*** TC*** GC*** Probe set 2 AA*C* AA*T*AA*G* AA*A* AC*A* AC*G* AC*T* AC*C* AG*T* AG*C* AG*A* AG*G* AT*G* AT*A*AT*C* AT*T* CA*T* CA*C* CA*A* CA*G* CC*G* CC*A* CC*C* CC*T* CG*C* CG*T*CG*G* CG*A* CT*A* CT*G* CT*T* CT*C* GA*G* GA*A* GA*C* GA*T* GC*T* GC*C*GC*A* GC*G* GG*A* GG*G* GG*T* GG*C* GT*C* GT*T* GT*G* GT*A* TA*A* TA*G*TA*T* TA*C* TC*C* TC*T* TC*G* TC*A* TG*G* TG*A* TG*C* TG*T* TT*T* TT*C*TT*A* TT*G* (Note 1: the probes themselves have reverse-complementarysequences) (Note 2: Star denotes any base)

FIG. 25 provides another depiction of redundancy introduced by theabove-described 2+4 encoding scheme. That is, the Base Sequence 331 isprovided with signals provided by the initial rounds of ligation 333provided above the sequence, while the signal provided by the 6^(th)round of ligation 335 provided below the sequence. As shown, the initialoffset ligation rounds provide signals 333 based on di-base readings;thus, the signal appears to fall between each nucleotide pair. That is,looking at the first two nucleotides of the sequence, T and C, a signal,depicted as 2 which could be considered to correspond with a color(e.g., Yellow). Redundancy is introduced because this signal isinsufficient to identify either of the bases. Rather, the systemrequires the second and third nucleotides, C and G, to be interrogatedby a second di-base probe which generates a second signal, shown as 3(e.g., Red). In view of these two readings and the probe chart above,the system can determine the identity of the second nucleotide, C.

FIG. 25 also depicts the additional layer of redundancy and enhancedaccuracy which is provided by interrogating data a second time with atleast one distinct probe set so as to generate a second signal 335 whichserves to introduce additional redundancy into the encoded data. Thatis, as shown below the Base Sequence 331, the second distinct probe set,in this case the (1, 3, 0, 3) 4-base specific probe set, an additionalsignal 335 is provided between, as shown herein, the 4^(th) and 5^(th)nucleotide, the 9^(th) and 10^(th) nucleotide, and the 14^(th) and15^(th) nucleotide. More specifically, the present example shows thatthe second probe set provides additional signals 1, 3, 0, between the4^(th) and 5^(th) nucleotides, 9^(th) and 10^(th) nucleotides, and the14^(th) and 15^(th) nucleotides, respectively.

FIG. 26 depicts another embodiment of a coding configuration 390 similarto the example shown in FIG. 11. In FIG. 26, however, coding of a basesequence 392 is depicted as being mediated by the two generator vectors392 (g₁ and g₂) so as to yield a code sequence 396. Thus, an example5-symbol window 400 yielding a 6-symbol code 402 can be considered to bea punctured convolutional coding scheme.

Punctured convolutional coding can represent a situation where certaincoded symbols resulting from a combination of two or more convolutionalcodings are unused. In the context of the example coding configuration390 shown in FIG. 26, symbols that would correspond to rounds 1 to 4with the generator vector g₂ (which would be overall rounds 7 to 10 ifperformed) can be viewed as being unused. Thus, of the possible 10symbols of code in window 402, symbols at positions 2, 4, 6, and 8 canbe considered to be “punctured.”

FIG. 27 shows another perspective 420 of the punctured convolutionalcoding scheme of FIG. 26. In certain situations, a sequence ofnucleotides being analyzed can be considered to be an example of aninformation sequence u (422); and a codeword x (426) can result fromcoding of u. An encoding operation can be represented as x=uG, where Grepresents a generator matrix 424 that yields, for example, a rate 5/6code. By implementing appropriate selection of generator vectors g₁ andg₂ as part of the generator matrix, encoding such as that shown in FIG.27 can be performed by sequencing systems such as SOLiD System.

FIG. 28 shows another example of a ligation-based sequencing operation.It will be understood that ligation based coverage of a templatesequence can facilitate error correcting coding, but is not arequirement for the purpose of introducing redundancy. In the context ofSOLiD System, a sequencing configuration 450 can include a templatestrand 452 having a base sequence 456 to be identified attached to a P1adapter 454. Such a strand can be anchored to a substrate such as a bead(not shown).

In FIG. 28, the example rounds 460 of measurements are depicted as beingfacilitated by primer offsets 462 and two groups of probes. For thepurpose of description of FIG. 28, five symbols of the ligation cyclesare shown. It will be understood that one or more universal symbols (notshown) can be present in such probes.

Round 1 is shown to be performed at an offset value of n−4, where n=5,and the interrogation can be performed via one of the two groups ofprobes (e.g., g₂ generated probes). As shown, Round 1 can include anumber of ligation cycles needed to cover the length of the templatesequence 456.

As shown, Round 2 can be performed at the same offset value (n−4) asthat for Round 1, and the interrogation can be performed via the otherof the two groups of probes (e.g., g₁ generated probes). As shown, Round2 can also include a number of ligation cycles needed to cover thelength of the template sequence 456. In certain embodiments, Rounds 1and 2 can be performed in a similar manner except the difference inprobes used.

As shown, Round 3 can be performed at an offset value of n−3, and theinterrogation can be performed via g₁ generated probes during a numberof ligation cycles. Round 4 can be performed at an offset value of n−2,and the interrogation can be performed via g₁ generated probes during anumber of ligation cycles. Round 5 can be performed at an offset valueof n−1, and the interrogation can be performed via g₁ generated probesduring a number of ligation cycles. Round 6 can be performed at anoffset value of n, and the interrogation can be performed via g₁generated probes during a number of ligation cycles.

As shown, ligation Cycle 2 of Round 1 yields a coded color depicted as472 a. Cycle 2 of Round 2 yields a coded color depicted as 472 b.Continuing, Cycle 2 of Round 3 yields a code 472 c, Cycle 2 of Round 4yields a code 472 d, Cycle 2 of Round 5 yields a code 472 e, and Cycle 2of Round 6 yields a code 472 f. Such rounds and ligation cycles canyield a color sequence 470 having redundant information suitable fordetection and correction of errors.

Referring back to 1A, the presently disclosed system further includes anECC Decoder 14 configured to receive the above-described encoded dataand decode such data while identifying and correcting errors without theneed to repeat any measurements.

The ECC Decoder 14 can utilize any of a number or combination ofalgorithms capable of generating corrected data from the above-describedencoding scheme. For example, the algorithm can be based on the BCJRAlgorithm (Bahl, Cocke, Jelinek, Raviv, “Optimal decoding of linearcodes for minimizing symbol error rate,” IEEE Transactions onInformation Theory, March 1974), the Viterbi Algorithm (Viterbi, “Errorbounds for convolutional codes and an asymptotically optimum decodingalgorithm,” IEEE Transactions on Information Theory, April 1967), theSoft Output Viterbi Algorithm (A Viterbi algorithm with soft-decisionoutputs and its applications,” Proceedings IEEE Conference on GlobalCommunications (GLOBECOM 1989), November 1989), Sequential Algorithms,such as: ZJ-Algorithm, Fano Algorithm, M-Algorithm, T-Algorithm,A*-Algorithm (Anderson, Mohan, “Sequential coding algorithms: a surveyand cost analysis,” IEEE Transactions on Information Theory, February1984), and Soft Output Sequential Algorithms, such as: LISS, BEAST,M*-BCJR, the entirety of each of these references being incorporatedherein by reference thereto. Those skilled in the art will appreciatethat various other such algorithms or combinations of algorithms arewithin the spirit and scope of the present disclosure.

In a preferred embodiment, the system utilizes the BCJR algorithm. TheBCJR algorithm, as depicted in FIG. 29, provides Bayesian inference ofbase probabilities. Bayesian inference is a method of statisticalinference in which some kind of evidence or observations are used tocalculate the probability that a hypothesis may be true, or else toupdate its previously-calculated probability. The term “Bayesian” comesfrom its use of the Bayes' theorem in the calculation process Inpractical usage, “Bayesian inference” refers to the use of a priorprobability over hypotheses to determine the likelihood of a particularhypothesis given some observed evidence; that is, the likelihood that aparticular hypothesis is true given some observed evidence (theso-called posterior probability of the hypothesis) comes from acombination of the inherent likelihood (or prior probability) of thehypothesis and the compatibility of the observed evidence with thehypothesis (or likelihood of the evidence, in a technical sense).Bayesian inference is opposed to frequentist inference, which makes useonly of the likelihood of the evidence (in the technical sense),discounting the prior probability of the hypothesis.

Referring again to FIG. 29, the BOR algorithm seeks to determine 4probabilities at each base position. That is, the algorithm isconfigured to determine the probability that the base at position i isA, the probability that the base at position i is C, the probabilitythat the base at position i is G, and the probability that the base atposition i is T. This determination is made at each base position. Asdetailed below, redundancy introduced into the encoded color data allowsfor the probability determination to take into account various readings,Thus, each result is based on multiple signals or measurements.

FIG. 30 provides another representation of the function of the BCJRalgorithm, That is, the BCJR algorithm 453 is configured to receivecolor likelihoods 451 for all base positions all i positions), andconvert such likelihoods into base probabilities 455 for all basepositions. As described above, due to the intricacies involved withencoding data, such as for example, in the context of DNA sequencing,exact signal determinations (e.g., color calls) may not be possible butrather the system may provide a series of signal likelihoods indicativeof the actual signal. As detailed below, the algorithm can be configuredto efficiently solve this problem by utilizing previous measurements aswell as breaking the problem into easier, more manageable problems.

In the context of DNA sequencing, referring to FIG. 31, the BCJRalgorithm can be thought of as establishing a grid 459 having 4 rowswith each row corresponding to each possible base (T, G, C, A), andhaving N columns corresponding to the encoded data points (e.g., thenumber of color likelihood signals to be evaluated). The algorithm canthen be considered to work through the grid 459 by evaluating eachpossible path in an effort to maximize the probability of each pathbased on the color likelihood data determined above. That is, the truebase sequence of T-C-G-T- . . . 457 is shown above the grid wherein eachbase is connected by the most likely color. That is, yellow (designatedas “2”) was determined to be the most likely color derived between theinitial 2 nucleotides, T and C.

In reviewing FIGS. 31-37B, please note that the signals are depicted asnumbers but could be considered colors. For example, “0” corresponds toBlue, “1” corresponds to Green, “2” corresponds to Yellow, and “3”corresponding to Red. Also note, as depicted in FIG. 31, the double-bandpositioned between the 5^(th) and 6^(th) nucleotides represents a doubleinterrogation event wherein the first band corresponds to a first signalgenerated by probes of a first probe set (e.g., a di-base specificprobe) and the second band corresponds to a second signal generated byprobes of a second probe set (e.g., a 4-base specific probe). At thisstep, the BCJR algorithm considers the probability of both signals whenmaximizing probability across the grid.

FIGS. 32 and 33 depict additional benefits of the BCJR algorithm.Namely, the algorithm is configured such that initial measurements donot need to be repeated when evaluating later base probabilities. Forexample, as shown in FIG. 32, the algorithm can determine probabilitiesup to a certain intermediate position (or “state”) in the grid and thenstart a “new” measurement from this position. For example, as shown, thealgorithm can maximize probabilities up to intermediate position “G”461. Then, as depicted in FIG. 33, position “G” 461 can then bedesignated as a starting point for a second set of calculations and thefinal result can be determined by summing those probabilities from thefar left of the grid to the G position 461 (463) and the sum of thoseprobabilities starting with the G position 461 and moving to the end ofthe graph (465). Thus, the BCJR algorithm does not have to start atnucleotide 1 every time the algorithm seeks to maximize the probabilityof a particular path which includes a later obtained color-likelihood.

FIG. 34 depicts another advantage of the BCJR algorithm. As shown, inaddition to (conceptually) starting from the far left of the grid andmoving left to right, the algorithm can also be configured to start atthe far right of the grid (e.g., the end of the sequence) and move rightto left. In contrast to moving left to right, where the first base istypically known, moving right to left requires additional steps whereeach of A T C G must be considered to be the first base. The ability toevaluate the date from left to right and right to left further providesfor efficient data analysis. Thus, any sum can be determined at anyposition by adding the sum determined from moving left to right 467 andthe sum determined from moving right to left 469.

FIGS. 35A-35E depicts some embodiments wherein the BCJR algorithm worksfrom left to right across the grid in maximizing base callprobabilities. That is, FIG. 35A shows that the first base is knownbecause, in some DNA sequencing systems, the first base can representthe last base of the adaptor sequencing prior to the beginning of theunknown template. In this example, this base is taken to be T. FIGS.35B-35E depicts the algorithm evaluating the probability that theidentity of the second base is T (FIG. 35B), G (FIG. 35C), C (FIG. 35D),and A (FIG. 35E). However, in view of the redundancy introduced in theencoding scheme, the first color likelihood does not provide enoughinformation to accurately identify the second base. Rather, the secondbase can only be identified after the algorithm has also maximized thevarious probabilities of second base based on the color-likelihoods ofthe signal generated between the 2^(nd) and 3^(rd) bases (as shown inFIG. 36A).

As such, the algorithm determines a base probability for b_(n) based oncolor likelihoods between b_(n−1) and b_(n) and between b_(n) andb_(n+1). The multiple interrogations of a single base is provided by theredundancy introduced by multiple ligation rounds with offset primers.Additionally, as shown in FIG. 36B, another layer of redundancy isprovided by the 6^(th) primer round with probes of a distinct probe set.As shown, the BJGR algorithm also maximizes each of the probe setsduring these “double band” steps.

As indicated above, the BCJR algorithm can also evaluate data from thefar right of the graph and move towards the beginning. FIGS. 37A and 37Bprovide representations of how these steps are initialized and carriedout. For example, FIG. 37A shows that, as opposed to moving left toright where the first base is typically known, moving from right to leftrequires that the algorithm consider possibilities where any of A, T, C,or G could be the “most probable” final base. Next, as shown in FIG.37B, all possible second (next to last) base positions are evaluatedrelative to all possible final base possibilities.

FIGS. 38A-38C provide additional representations of the system's decoderutilizing 2+4 encoding (i.e., 5 ligation rounds utilizing primer offsetsand di-base codes from probe family 1 in addition to a 6^(th) ligationround at a previously interrogate primer offset with probes of a secondprobe family). FIG. 38A provides a state graph wherein base b₀ is thelast base of the adaptor sequence. Bases b₁-b_(n) are unknown and mustbe determined from color measurements. As sown, the graph consists of Nsections.

FIG. 38B provides one section N of the state graph of FIG. 38A. At thebeginning and end of each section there are 64 states, corresponding tobase triplets. Each starting state connects to 4 ending states with abranch. A branch corresponding to a quadruplet of bases [b_(i), b_(i+1),b_(i+2), b_(i+3)], when it connects state [b_(i), b_(i+1), b_(i+2)] tostate [b_(i+1), b_(i+2), b_(i+3)].

FIG. 38C shows a subsection of one section of the state graph. As shown,a branch that corresponds to quadruplet of bases [b_(i), b_(i+1),b_(i+2), b_(i+3)], has an expected color in (1,1) probe set and anexpected color in the (1,3,0,3) probe set. As detailed above, theexpected colors are used to assign a metric to the branch, based on howwell do measurements support that particular branch. Note, themeasurements for the (1,3,0,3) probe set are only available in everyfifth graph section.

FIG. 39 shows error correction performance curves 480 for various K-basecodes and primer rounds in ligation-based measurement system model. Thecurves 480 are plotted with base call error rate on the vertical axisand color call error rate on the horizontal axis. Data for such curvescan be generated via the example feedback system described herein inreference to FIG. 22.

In FIG. 39, the four example curves represent error correctionperformances for the configurations listed in Table 1. Moreparticularly, curve 488 represents the 5-base code and 6-primer-roundsconfiguration described in reference to FIG. 21. Curve 484 representsthe 3-base code and 6-primer-rounds configuration; curve 486 representsthe 4-base code and 6-primer-rounds configuration; and curve 490represents the 5-base code and 7-primer-rounds configuration.

Also shown in FIG. 39 is a reference curve 482 where coding and decodingare not performed. For such a curve, a color call error translatesdirectly into a base call error; and thus provides a good performancereference with respect to detection of color call errors and correctingsuch error to what the base call should be. As shown, the errorcorrected curves perform substantially better than the uncorrected curve482 when the color call error rate is relatively low. When the colorcall error rate is relatively high, the corrected curves perform worse.It is believed that such an effect is likely due to a fact that coding(and in particular with large Hamming distance coding) for errorcorrection has a tendency to spread out the likelihood of errors. Withsuch spreading, correction of such errors can benefit significantly whenthe likelihood of error is relatively low. On the other hand, spreadingout of relatively high error rates can result in the correction makingthe errors worse.

In certain embodiments, and as apparent in FIG. 39, coding for errorcorrection can be implemented in situations where the expected colorcall rate is relatively low. Further, the value for K (number of symbolsin the code) can be selected based on the expected color call rate.Also, in the context of the two 5-base codes 488 and 490, the7-primer-round case (490) displays significantly better performance thanthe 6-primer-round case (488). As described herein, such performanceenhancement can be weighed against other factors.

As described herein, encoding of data can be achieved via configuring ofthe probes in certain manners. Introduction of redundancy to the encodeddata can be achieved via, for example, performing additionalmeasurements. Because encoding and redundancy-introduction are integralparts of the probes and measurements, decoding and resulting errordetection and correction steps do not necessarily require a referencesequence. Such decoding can be performed simply based on the encodingprocess.

FIG. 40 shows another example of how such decoding step can beintegrated into sequencing systems such as the SOLiD System. A decodingconfiguration 500 can include a decoder 504 that receives input datafrom a first analysis component 502. Such an input data can includecolor likelihoods of the detected dyes. The decoder 504 can decode theencoded sequence of color likelihoods and generate base probabilities,and such base probabilities can be provided to a second analysiscomponent 506 for further analysis.

In certain embodiments, such base probabilities can also be providedback to the first analysis component (via arrow 509). If theprobabilities are such that the decoder considers the result erroneous,the feedback 509 can allow correction of the color likelihood sequenceby the first analysis component 502. Again, such determination of baseprobability error/ambiguity and any correction/resolution to the colorlikelihoods do not require reliance on any reference other thanknowledge of the encoding scheme.

FIG. 41 shows a process 510 that can be implemented to provide thedecoding feedback functionality as described in reference to FIG. 23. Ina process block 512, coded sequence of color likelihoods can beobtained. As described herein, such coded sequence can include at leastsome redundant information. In a process block 514, the sequence ofcolor likelihoods can be decoded. In a process block 516, the processcan include performing one or more feedback operations based on baseprobabilities. In a process block 518, the process can include derivingmost likely base calls and a base quality value from base probabilities.

The presently disclosed system and methods can include various otherembodiments capable of contributing to improved polynucleotidesequencing accuracy. As described above, ultra-high throughput nextgeneration sequencing (NGS) technologies, such as the SOLiD platform,provide the ability to sequence genomes quickly and cheaply. NGS systemstypically read many more DNA fragments and produce shorter read lengthsthan traditional sequencing systems. Because it is generally consideredimpractical to generate de novo assembly from short reads if the errorrate is greater than about one percent, NGS is mostly used for genomere-sequencing, e.g., finding SNPs and other differences in a humansample compared to the reference.

Short read NGS technologies coupled with error correction techniques canallow de novo assembly of previously unknown genomes. In one embodiment,one such error correction technique is based on an alignment of multiplereads without explicit pair-wise comparison. Repeating units of knucleotides from portions of reads are used for fast hash-basedalignment.

In the error correction technique, a set of reads, R, is corrected. If aparticular k-mer appears at least m times in R, then the particulark-mer is included into a set of frequent k-mers called a spectrum. Errorcorrection is performed by first examining all reads in the set R fork-mers that are close to being error-free k-mers. An error-free k-mer isdefined as a k-mer having the exact sequence of nucleotides found in theparticular k-mer from the spectrum. An error-free k-mer is also called asolid k-mer. An error-free or solid read is defined as a read thatinclude only solid k-mers. Each read in the set R is examined todetermine if by mutating a few nucleotides in the read an error-freek-mer can be formed from a k-mer that is close to being error-free. If amutation is found that results in producing an error-free k-mer, themutation is made and the read is error corrected. A mutation is made bysubstituting a nucleotide with one of three other possible nucleotides.

In certain embodiments, spectral alignment error correction (SAEC) isused to decrease the color call rate of an NGS system. Some NGS systems,such as the SOLiD™ platform, use two base encoding, as described above.Applying error correction to the color calling in a two base encodingsystem is more advantageous that applying error correction to a one basesystem. For example, a one color difference in a two base encodingsystem is almost always an error, while a one base difference in a onebase system is often a duplication in the genome.

FIG. 42 is schematic diagram of a system 2500 for error correcting DNAsample reads using SAEC, in accordance with certain embodiments. System2500 includes DNA sequencer 2510 and processor 2520. DNA sequencer 2510can include, but is not limited to including, detection zone 2512,optics 2514, and detector 2516. DNA sequencer 2510 can be, but is notlimited to, an NGS system such as the SOLiD™ platform. Processor 2520can be, but is not limited to, a computer, microprocessor, or any devicecapable of sending and receiving control signals and data from DNAsequencer 2510 and processing data.

DNA sequencer 2510 and processor 2520 perform SEAC on color call DNAsample reads. In the context of certain fluorescence-based sequencingprocesses, a color call DNA sample read is a sequence fluorescencecolors that represent the sequence of the DNA produced by DNA sequencer2510. DNA sequencer 2510 analyzes a plurality of DNA samples andproduces a plurality of reads from the plurality of DNA samples.Processor 2520 is in communication with the DNA sequencer 2510 andperforms a number of steps.

Processor 2520 obtains the plurality of reads from the DNA sequencer.Processor 2520 then examines the plurality of reads for a sequence ofconsecutive color calls of length k that appear in the plurality ofreads at least m times. As described above, if a particular sequence ofconsecutive color calls of length k, a k-mer, appears at least m timesin the plurality of reads, the k-mer is said to belong to a spectrum. Asa result, processor 2520 examines the plurality of reads for spectrumconstruction.

Finally, a spectrum is constructed, processor 2520 attempts errorcorrection. In other words, if a spectrum is found, processor 2520attempts error correction. Processor 2520 analyzes each read of theplurality of reads. For each read, processor 2520 examines each k-merthat does not belong to the spectrum and tries to mutate the color callin it. Color calls are examined based on their quality values. Processor2520 starts with a color call with the lowest quality value and selectseach succeeding or next color call corresponding to an increasingquality value, for example. Processor 2520 changes or corrects eachcolor call it examines if the change produces a corrected sequence oflength k that includes the changed color call and matches the sequenceof consecutive color calls of length k in the spectrum. In other words,processor 2520 attempts to substitute color calls in k-mers of readsthat almost match the k-mer of the spectrum, in order to maximize thenumber of k-mers that exactly match the k-mer of the spectrum.

In certain embodiments, the spectral parameters k and m are optimizedusing experimental data. For example, applying SAEC to bacterial genomesresults in an optimal value of 17 for k. An optimal value for m isdependent on the coverage. For example, a coverage of 600 times resultsin an optimal value for m of 8, while a coverage of 300 times results inan optimal value for m of 5.

In certain embodiments, a probabilistic heuristic can be used todetermine spectral parameters. For example, a probabilistic heuristiccan be used to determine the optimal k-mer size. Also, a numericalanalysis method can be used to find the most optimal division between aset of trustable and non-trustable k-mers in the spectrum. For example,for a given estimate of genome size, L, the number of correct k-merscannot be larger than L. If both strands of DNA are considered, then thenumber of correct k-mers cannot be larger than 2*L. It is assumed that Lis estimated with an accuracy of +/−20%, therefore, the top 2*L(1+/−0.2)high frequency k-mers are targeted for trustable values. If on thesegment 2*L*0.8 to 2*L*1.2, for example, there is a k-mer frequencypoint, such that there is an exponential increase in the number ofk-mers with lower frequency, then this point is an optimal divisionbetween sets of trustable and non-trustable k-mers.

In certain embodiments, DNA sequencer 2510 is a two base encoded DNAsequencer. As described above, SAEC is particularly advantageous for DNAsequencers that provide two base encoding, such as the SOLiD™ platform.

In certain embodiments, processor 2520 does not change the color call ifan adjacent color call was previously changed. To prevent overcorrectionand generation of chimeric reads, correction in two adjacent positionsis avoided.

In certain embodiments, processor 2520 examines the plurality of readsfor a sequence of consecutive color calls of length k that appear in theplurality of reads at least m times such that the sequence includescolor calls having quality values above a threshold value. In otherwords, quality values are used to calculate the spectrum so thatsystematic errors, or errors that are frequent in the same position, donot go into the spectrum.

In certain embodiments, multiple rounds of error correction are used todecrease error rates. For example, processor 2520 examines the pluralityof once corrected reads for a second sequence of consecutive color callsof length k that appear in the plurality of reads at least m times. Ifthe second sequence of consecutive color calls of length k appears inthe plurality of reads at least m times, processor 2520 attempts errorcorrection. In other words, after a second spectrum is constructed,processor 2520 attempts error correction. Processor 2520 analyzes eachread of the plurality of reads. For each read, processor 2520 examineseach k-mer that does not belong to the spectrum and tries to mutate eachcolor call in it. Color calls are examined based on their qualityvalues. Processor 2520 starts with a color call with the lowest qualityvalue and selects each succeeding next color call according to anincreasing quality value, for example. Processor 2520 changes orcorrects each color call it examines if the change produces a secondcorrected sequence of length k that includes the changed color call andmatches the second sequence of consecutive color calls of length k usedto define the second spectrum.

In certain embodiments, color calls are examined based on their numberof spectral votes in addition to their quality values. A color callreceives a spectral vote if a mutation in that color call makes a seedbelong to the spectrum. A seed is a portion of a spectrum, for example.When spectral votes are used, processor 2520 starts with a color callwith the lowest quality value and most spectral votes and selects eachsucceeding next color call according to an increasing quality value anddecreasing spectral vote count.

In certain embodiments, processor 2520 combines spectral votes with theprobability of error associated with quality values and corrects mostlikely errors, thus avoiding overcorrection. For example, for a certainmutation with v votes in the read position and with quality value q, anadjusted number of votes is equal to v*(1+10*P_(error)(q)). P_(error) isthe probability of error in a position with quality value q.

FIG. 43 is an exemplary portion of an alignment 2600 of DNA color callreads, in accordance with certain embodiments. Alignment 2600 includestwelve color call reads. Between dashed line 2610 and dashed line 2620 asequence of consecutive color calls of length 17 appears in the twelvereads 8 times. This sequence is shown in alignment 2600 as a sequence of17 consecutive white color calls. The twelve reads that include this17-mer sequence belong to a 17-mer spectrum. The remaining four reads inalignment 2600, read 2631, read 2632, read 2633, and read 2634, eachinclude one black color call between dashed line 2610 and dashed line2620. In certain embodiments, SAEC is used to change the one black colorcall in read 2631, read 2632, read 2633, and read 2634 to a white colorcall.

FIG. 44 is a flowchart showing a method 2700 for error correcting DNAsample reads using SAEC, in accordance with certain embodiments.

In step 2710 of method 2700, a plurality of DNA samples is analyzed anda plurality of reads from the plurality of DNA samples is produced usinga DNA sequencer.

In step 2720, the plurality of reads from the DNA sequence is obtainedusing a processor in communication with the DNA sequencer.

In step 2730, the plurality of reads is examined for a sequence ofconsecutive color calls of length k that appear in the plurality ofreads at least m times using the processor.

In step 2740, it is determined if the sequence of consecutive colorcalls of length k appears in the plurality of reads at least m times.

In step 2750, if the sequence of consecutive color calls of length kappears in the plurality of reads at least m times, for each read of theplurality of reads and for each color call of the each read, a colorcall with a lowest quality value is selected as the starting color call,a next color call is selected that has a corresponding increasingquality value, and each color call that is selected is changed if thechange produces a corrected sequence of length k that includes thechanged color call and matches the sequence of consecutive color callsof length k using the processor.

In certain embodiments, a computer program product includes a tangiblecomputer-readable storage medium whose contents include a program withinstructions being executed on a processor so as to perform a method forerror correcting DNA sample reads using SAEC. This method is performedby a system of distinct software modules.

FIG. 45 is a schematic diagram of a system 2800 of distinct softwaremodules that performs a method for error correcting DNA sample readsusing SAEC, in accordance with certain embodiments. System 2800 includesdetection module 2810 and error correction module 2820. Detection module2810 and error correction module 2820 perform a number of steps.

Detection module 2810 obtains a plurality of reads from a DNA sequencerthat analyzes a plurality of DNA samples and produces the plurality ofreads from the plurality of DNA samples.

Error correction module 2820 examines the plurality of reads for asequence of consecutive color calls of length k that appear in theplurality of reads at least m times.

If the sequence of consecutive color calls of length k appears in theplurality of reads at least m times, for each read of the plurality ofreads and for each color call of the each read, error correction module2820 starts with a color call with a lowest quality value, selects anext color call with an increasing quality value, and changes each colorcall, if the change produces a corrected sequence of length k thatincludes the changed color call and matches the sequence of consecutivecolor calls of length k.

In some embodiments, a system for error correcting polynucleotide (e.g.,DNA) sample reads using spectral alignment error correction is provided.The system can include a sequencer (e.g., a two-base encoded sequencer)that analyzes a plurality of polynucleotide samples and produces aplurality of reads from the plurality of DNA samples. The system canalso include a processor in communication with the DNA sequencer thatcan obtain the plurality of reads from the DNA sequencer, examine theplurality of reads for a sequence of consecutive color calls (orlikelihoods) of length k (e.g., 17) that appear in the plurality ofreads at least m times, and if the sequence of consecutive color callsof length k appears in the plurality of reads at least m times, canperform further analysis. For example, for each read of the plurality ofreads and for each color likelihood of the each read the processor canstart with a color likelihood with a lowest quality value, select a nextcolor likelihood with an increasing quality value, and change each colorlikelihood if the change produces a corrected sequence of length kcomprising each color call that matches the sequence of consecutivecolor calls of length k.

In some embodiments, the processor can start with a color call having ahighest number of spectral votes in addition to a lowest quality valueand select a next color call with a decreasing number of spectral votesin addition to an increasing quality value. In some embodiments, theprocessor does not change each color call if an adjacent color call waspreviously changed. In some embodiments, the sequence of consecutivecolor calls of length k can includes color calls having quality valuesabove a threshold value.

In some embodiments, the processor can be configured to examine theplurality of reads for a second sequence of consecutive color calls oflength k that appear in the plurality of reads at least m times.Further, if the second sequence of consecutive color calls of length kappears in the plurality of reads at least m times, for each read of theplurality of reads and for each color call of the each read theprocessor starts with a color call with a lowest quality value. Theprocessor can also select a next color call with an increasing qualityvalue, and change each color call if the change produces a secondcorrected sequence of length k comprising each color call that matchesthe second sequence of consecutive color calls of length k.

Various methods for error correcting polynucleotide sample reads usingspectral alignment error correction are also provided herein. The methodcan include, for example, analyzing a plurality of polynucleotide (e.g.,DNA) samples and producing a plurality of reads from the plurality ofsamples using a sequencer. The method can also include obtaining theplurality of reads from the DNA sequencer using a processor incommunication with the DNA sequencer and examining the plurality ofreads for a sequence of consecutive color calls of length k that appearin the plurality of reads at least m times using the processor. In someembodiments, if the sequence of consecutive color calls of length kappears in the plurality of reads at least m times, for each read of theplurality of reads and for each color call of the each read startingwith a color call with a lowest quality value, the method includesselecting a next color call with an increasing quality value, andchanging each color call if the change produces a corrected sequence oflength k comprising each color call that matches the sequence ofconsecutive color calls of length k using the processor.

Various embodiments of a computer program product are also disclosedherein. For example, the computer program product can include a tangiblecomputer-readable storage medium whose contents include a program withinstructions being executed on a processor so as to perform a method forerror correcting DNA sample reads using spectral alignment errorcorrection. Various embodiments of such a method are disclosed herein.For example, the method can include providing a system having distinctsoftware modules, and wherein the distinct software modules comprise adetection module and an error correction module. The method can alsoinclude obtaining a plurality of reads from a sequencer that analyzes aplurality of polynucleotide (e.g., DNA) samples and produces theplurality of reads from the plurality of DNA samples using a detectionmodule. The method can also include examining the plurality of reads fora sequence of consecutive color calls of length k that appear in theplurality of reads at least m times using the error correction module.In some embodiments, if the sequence of consecutive color calls oflength k appears in the plurality of reads at least m times, for eachread of the plurality of reads and for each color call of the each readstarting with a color call with a lowest quality value, the method caninclude selecting a next color call with an increasing quality value,and changing each color call if the change produces a corrected sequenceof length k having each color call that matches the sequence ofconsecutive color calls of length k using the error correction module.

Although the above-disclosed embodiments have shown, described, andpointed out the fundamental novel features of the invention as appliedto the above-disclosed embodiments, it should be understood that variousomissions, substitutions, and changes in the form of the detail of thedevices, systems, and/or methods shown may be made by those skilled inthe art without departing from the scope of the invention. Consequently,the scope of the invention should not be limited to the foregoingdescription, but should be defined by the appended claims.

All publications and patent applications mentioned in this specificationare indicative of the level of skill of those skilled in the art towhich this invention pertains. All publications and patent applicationsare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

1. A method for obtaining sequence information about a polynucleotide,comprising: configuring a template polynucleotide to allow interrogationof K nucleotides; and interrogating said nucleotides to yieldmeasurements of M detectable characteristics, wherein said measurementsinclude data representative of one or more of said K nucleotides anddata that is redundant regarding a characteristic of, or a sequencerelationship between or among, one or more of said K nucleotides,wherein said redundant data is suitable for detection of errors orambiguities and/or correction of errors or resolution of ambiguities. 2.The method of claim 1, wherein said redundant data is suitable fordetection and/or correction or errors.
 3. The method of claim 1, whereinsaid redundant data is suitable for detection and/or resolution ofambiguity.
 4. The method of claim 1, wherein said redundancy is achievedby having a quantity N^(M) greater than a quantity L^(K), each of said Knucleotides being one of L types, each of said M detectablecharacteristics being one of N types.
 5. The method of claim 4, whereinsaid quantity L comprises quantity of 4 corresponding to nucleotidetypes A, C, G, and T.
 6. The method of claim 5, wherein said redundancyis achieved by selecting said quantity N.
 7. The method of claim 5,wherein said redundancy is achieved by selecting said quantity M.
 8. Themethod of claim 1, wherein said K nucleotides are interrogated by one ormore probes, each of said probes having one or more detectable labelsattached thereto.
 9. The method of claim 8, wherein said interrogationcomprises a ligation-based process where a number of unique probingsteps are performed, such that said redundancy is achieved by having aquantity N^(M) greater than a quantity L^(K), each of said K nucleotidesbeing one of L types, each of said M detectable characteristics beingone of N types, said quantity M represented as M=K*S/P where Srepresents said number of unique probing steps and P represents a numberof primer offsets allowed in said hybridization and/or ligation-basedprocess.
 10. The method of claim 8, wherein said redundancy is achievedby selecting said quantity N.
 11. The method of claim 10, wherein saidquantity N is based on N different types of a single label.
 12. Themethod of claim 11, wherein said quantity N has a value greater than orequal to
 4. 13. The method of claim 10, wherein said quantity N is basedon different types of a plurality of labels.
 14. The method of claim 9,wherein said redundancy is achieved by selecting said quantity S. 15.The method of claim 14, wherein said quantity S comprises a number ofrounds of probing by a group of probes based on said P offsets, eachprobe of said group having a detectable label assigned to it based on aunique scheme.
 16. The method of claim 15, wherein said quantity Sfurther comprises one or more rounds of additional probing by one ormore additional groups of probes, each of said one or more additionalrounds being performed at a primer offset already probed by said groupof probes, each of said one or more additional groups having a uniquescheme for assigning detectable labels to its probes.
 17. The method ofclaim 1, further comprising decoding of said measurements so as todetermine whether said measurement includes any error.
 18. The method ofclaim 17, further comprising performing an error correction based onsaid detected error and said redundancy. 19.-32. (canceled)
 33. Apolynucleotide sequencing system, comprising: a detection zoneconfigured to facilitate interrogation of a template polynucleotide; adetector configured to obtain measurements associated with saidinterrogation, said measurements including data representative of saidpolynucleotide and having redundancy; and an error detection componentconfigured to allow detection of an error resulting from interpretationof said data.
 34. The system of claim 29, further comprising an errorcorrection component configured to allow correction of said detectederror based at least in part on said redundancy.